Research on the Preparation of MIL-68(Fe)/g-C3N4 Materials for the Simultaneous Photocatalytic Removal of Cr(Ⅵ) and Tetracycline

doi:10.21203/rs.3.rs-5018356/v1

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Research on the Preparation of MIL-68(Fe)/g-C₃N₄ Materials for the Simultaneous Photocatalytic Removal of Cr(Ⅵ) and Tetracycline

https://doi.org/10.21203/rs.3.rs-5018356/v1

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Heterostructure composite materials are widely used in environmental remediation due to their excellent photocatalytic activity. This study synthesized MIL-68(Fe) on the surface of g-C₃N₄ using an in-situ synthesis method to form a composite material with high specific surface area and superior photocatalytic performance. The morphology, crystal structure, chemical composition, and photocatalytic properties of this material were characterized in detail. Experimental results showed that MIL-68(Fe)/g-C₃N₄ composite material achieved a 99.9% removal rate of Cr(Ⅵ) and a 90% degradation rate of tetracycline within 80 minutes. Compared with MIF-68(Fe) or g-C₃N₄ alone, the tetracycline degradation rate and Cr(Ⅵ) reduction rate of the composite material increased significantly, which greatly enhanced its photocatalytic activity. This enhancement can be attributed to the composite material exhibiting nearly five times higher specific surface area than MIL-68(Fe) alone. Additionally, after doping with g-C₃N₄, the UV absorption of MIL-68(Fe)/g-C₃N₄ composite material decreased, and the absorption edge showed a red shift, indicating that MIL-68(Fe)/g-C₃N₄-3 may have better visible light response. This work demonstrates that MIL-68(Fe) is an efficient photocatalyst and provides new insights for subsequent research in the field of photocatalytic degradation using MIL-68(Fe).

MIL-68(Fe)

Photocatalysis

Dye

Tetracycline

Graphitic Carbon Nitrides (g-C3N4)

Hexavalent chromium (Cr(Ⅵ)) is a highly toxic and carcinogenic substance that has attracted worldwide attention due to its environmental and health impacts(Du et al. 2019; Gong et al. 2010). It is crucial to remove chromium from wastewater to prevent it from spreading and contaminating drinking water sources (Gheju et al. 2008; Gheju and Balcu 2010). Cr(Ⅵ) is a common toxic heavy metal and is considered a teratogen and mutagen in ecosystems. Numerous studies have demonstrated the toxic effects of excessive chromium exposure on aquatic organisms. Various methods, such as chemical reduction, adsorption, ion exchange, precipitation, and photocatalytic reduction, have been explored to remove Cr(Ⅵ)(Attia et al. 2010; Kucic et al. 2017) Different oxidation states of chromium exhibit different levels of biological toxicity: Cr(III) is much less toxic than Cr(Ⅵ), and it is more challenging to remove Cr(Ⅲ) using conventional chemical and physical techniques(Kumar et al. 2008; Sharma et al. 2016; Y. Wang et al. 2017). Photocatalytic reduction has emerged as a novel water treatment method due to its superior performance(Mahlambi et al. 2015; Serpone and Emeline 2012). However, the effectiveness of photocatalysts is sometimes limited by their narrow absorption activation wavelength range.

With the rapid development of the modern economy, the use of various medications for disease treatment has increased, raising concerns about the widespread presence of pharmaceutical compounds and residues(Cheng et al. 2023). Among the treatment methods, antibiotics are one of the most commonly used options and are considered highly persistent emerging pollutants. When antibiotic residues enter water sources, they are believed to have significant negative environmental impacts(Ben et al. 2019). Commonly used pharmaceuticals include ibuprofen, tetracycline, metformin, and acetylsalicylic acid (Balakrishnan et al. 2022). Tetracycline can enter water sources through various pathways and contaminate water flows. Unlike other pollutants, tetracycline does not have multiple uses, necessitating immediate action to address the contamination caused by other pharmaceuticals

(Zhou et al. 2012).

Metal-organic frameworks (MOFs) have become a new research hotspot in the past decade due to their unique spatial structures, ultra-high specific surface areas, and large pore volumes. To date, MOFs have been successfully applied in gas storage, adsorption, separation, catalysis, and drug delivery(Horcajada et al. 2006; J.-R. Li et al. 2009; Ma et al. 2009; Rowsell and Yaghi 2005). Since the first report on the photocatalytic activity of MOF-5, various MOF materials, such as Ni-MOF(Mahata et al. 2006)、MIL-101(Cr)(Wen et al. 2016)、NH2-MIL-68(In)(Liang, Shen, et al. 2015)、NH2-UiO-66(Zr)(Shen et al. 2013) have been used as semiconductor photocatalysts. Among MOF materials, Fe-based MOFs are considered promising photocatalysts due to their high chemical stability, non-toxicity, low cost, and good photosensitivity. In recent years, a series of Fe(Ⅲ)-based MOFs, such as MIL-53(Fe), MIL-68(Fe), MIL-88(Fe), MIL-100(Fe), and MIL-101(Fe), have been reported for photocatalytic dye degradation or Cr(Ⅵ) reductio(Jing et al. 2017; Liang, Jing, et al. 2015; Liu et al. 2018; Ma et al. 2009; C.-C. Wang et al. 2016). Despite the significant progress in MOF photocatalysts over the past decades, their photocatalytic performance is still limited by factors such as low solar energy conversion efficiency, low conductivity, and rapid e-/h + recombination(Giannakoudakis et al. 2017). Consequently, many studies have attempted to construct heterostructures using other materials to overcome these shortcomings. These strategies aim to regulate the photocatalytic performance of MOFs, but they often require expensive noble metals and complex preparation processes, and some composite materials still need scavengers to achieve Cr(Ⅵ) reduction, which greatly limits their practical applications.

MIL-68(Fe), first reported by the Gérard Férey team, is an iron-based metal-organic framework structure composed of Fe(OH)2O4 corner-sharing octahedral chains connected by BDC (benzene-1,4-dicarboxylate)(Fateeva et al. 2010). Compared with other Fe-MOFs, reports on the photocatalytic applications of MIL-68(Fe) are relatively scarce (D. Wang et al. 2015).Fenfen Jing(Jing et al. 2017) reported the photocatalytic reduction of Cr(Ⅵ) under visible light using MIL-68(Fe), Additionally, Ruowen Liang(Liang et al. 2021) reported modifying MIL-68(Fe) with Ag nanoparticles (Ag NPs) and subsequently depositing AgBr particles on Ag@MFe hybrids to form the final ternary sandwich-like hierarchical AgBr-Ag@MFe photocatalyst. photocatalytic degradation of Rhodamine B was 9.2 times higher than that of the original MIL-68(Fe). In fact, MIL-68(Fe) has also demonstrated excellent photocatalytic performance, but its larger micro-sized dimensions compared to other Fe-MOFs hinder its photocatalytic applications to some extent.

Since the discovery of monolayer graphene in 2004, 2D nanomaterials have increasingly attracted attention for their applications in pollutant treatment due to their atomically thin layered structures and fully exposed adsorption surfaces. In recent years, the novel graphitic carbon nitride (g-C₃N₄) photocatalyst has shown great potential in water pollutant treatment due to its excellent chemical and thermal stability, semiconductor properties, and relatively suitable band positions(K. Li et al. 2016; Mao et al. 2013; “Markedly enhanced visible-light photocatalytic H2 generation over g-C3N4 nanosheets decorated by robust nickel phosphide (Ni12P5) cocatalysts - Dalton Transactions (RSC Publishing)” n.d.). However, the original g-C₃N₄ has certain limitations, such as low specific surface area and easy recombination of photogenerated carriers, which result in insufficient light utilization. Therefore, many studies have focused on enhancing the performance of g-C₃N₄ by improving the separation efficiency of photogenerated carriers and increasing the specific surface area of nanosheets to obtain stable g-C₃N₄ nanosheets. Various research results have confirmed the effectiveness of these methods, such as bulk exfoliation, defect creation, and doping modification. Another effective strategy is to couple g-C₃N₄ with other semiconductors to form heterostructure composites, allowing rapid separation of photogenerated charges and improving photocatalytic activity(“Comparing Two New Composite Photocatalysts, t-LaVO4/g-C3N4 and m-LaVO4/g-C3N4, for Their Structures and Performances | Industrial & Engineering Chemistry Research” n.d.). Additionally, MOFs' high surface area and multiple active sites are particularly advantageous when forming composites with g-C₃N₄(“Oxidized g-C3N4 Nanospheres as Catalytically Photoactive Linkers in MOF/g‐C3N4 Composite of Hierarchical Pore Structure - Giannakoudakis − 2017 - Small - Wiley Online Library” n.d.). Specifically, Fe-based MOFs are favored by researchers due to their porosity and semiconductor characteristics (C. Zhang et al. 2021). The presence of Fe-oxygen clusters with significant visible light range absorption makes Fe-based MOFs potential materials for enhancing the photocatalytic activity of g-C₃N₄.(Pan et al. 2021). For example, Pan et al. studied the composite of g-C₃N₄ and MIL-53(Fe), which showed good photocatalytic degradation of high-concentration tetracycline(Pan et al. 2021); Zhao et al. combined g-C₃N₄ with MIL-101(Fe), and the composite material formed a direct Z-scheme heterojunction with appropriate band alignment, enhancing light absorption and effectively separating carriers, resulting in better photocatalytic performance than individual materials(Zhao et al. 2020).

In this work, we used 2D g-C₃N₄ materials as a support to construct heterostructures with MIL-68(Fe), improving the photocatalytic performance of MIL-68(Fe). Using a simple in situ synthesis method, MIL-68(Fe) was grown on g-C₃N₄ as a support framework, forming a high-specific-surface-area crystal structure. The morphology, crystal structure, and chemical composition of the prepared composite materials were characterized. The MIL-68(Fe)/g-C₃N₄ hybrid was used for the photocatalytic reduction of Cr(Ⅵ) and the simultaneous degradation of tetracycline.

1.1 Materials

Urea (CH₄N₂O) was provided by Sinopharm Chemical Reagent Co., Ltd. Ferric chloride hexahydrate (FeCl₃·6H₂O), potassium dichromate (K₂Cr₂O₇), ammonium oxalate ((NH₄)₂C₂O₄), and N,N-dimethylformamide (C₃H₇NO) were provided by Tianjin Damao Chemical Reagent Factory. Diphenylcarbazide (C₁₈H₃7_N) was provided by Tianjin Fuchen Chemical Reagent Factory. Acetone (C₃H₆O) was provided by Chengdu Kelong Chemical Reagent Co., Ltd. Concentrated sulfuric acid (H₂SO₄) and concentrated hydrochloric acid (HCl, 36%) were provided by Guangzhou Chemical Reagent Co., Ltd. Hydrochloride tetracycline (C₂₂H₂₅ClN₂O₈) and terephthalic acid (C₈H₆O₄) were provided by Aladdin Reagent Co., Ltd. Hydrofluoric acid (HF, 40%) was provided by Tianjin Baishi Chemical Co., Ltd. All materials were used as received without further purification.

1.2 Synthesis of Photocatalytic Samples

The g-C₃N₄ 2D material was synthesized based on an improved version of a previously reported strategy(Tang et al. 2022). 30 g of urea was placed in a crucible and heated to 550°C at a rate of 2.5°C/min, then calcined at high temperature for 4 hours, followed by natural cooling to obtain g-C₃N₄.

MIL-68(Fe)/g-C₃N₄ was synthesized using a one-step solvothermal method. 1.94 g of FeCl3·6H2O and 2.39 g of H2BDC were mixed with 72 mL of DMF, then 360 µL of HF (5 mM) and 360 µL of HCl (1 mM) were added. After stirring for 2 minutes, a predetermined amount of g-C₃N₄ was added and stirred for 30 minutes to fully mix. The solution was transferred to a 100 mL Teflon-lined autoclave and heated at 100°C for 5 days, followed by cooling to room temperature. The reaction product was collected by centrifugation at 5000 rpm, and washed three times with DMF and acetone, respectively. The resulting yellow solid was dried overnight at 60°C to remove residual acetone. The MIL-68(Fe)/g-C₃N₄ heterojunction composites were named MIL-68(Fe)/g-C₃N₄-X (X = 12345) according to the amount of g-C₃N₄ (450, 150, 50, 10, 2.5 mg) used.

1.3 Characterization

Catalyst samples were ultrasonically dispersed in water and observed using a Zeiss scanning electron microscope (SEM) from Germany. Elemental distribution images of the samples were analyzed using an X-Max20 scanning electron microscope energy dispersive X-ray (EDX) mapping. The crystal structure of the samples was determined by powder X-ray diffraction (XRD) using a Bruker D8 Advanced Diffractometer System, with a Cu Kα (λ = 1.5406 Å) radiation source, operating voltage of 40 kV, and current of 40 mA, scanning at a speed of 0.1 s/step with a step size of 0.02 degrees. Fourier transform infrared spectroscopy (FTIR) was used to determine the surface chemical structure of the catalyst, tested with a Thermo Fisher Scientific instrument. The surface chemical composition of the samples was measured using X-ray photoelectron spectroscopy (XPS) with a Thermo Scientific NEXSA. The optical absorption properties of the samples were tested using a Cary 60 spectrophotometer, from which the band gap of the material was calculated. The specific surface area and pore size distribution of the samples were measured using an ASAP TriStar II 3flex, and the specific surface area values were obtained using the Brunauer-Emmett-Teller (BET) method. The thermal degradation and thermal stability of the samples were analyzed using a TA SDT Q600 thermal analyzer, with nitrogen as the protective gas, heating range from 25 to 800°C, and a heating rate of 10°C/min. UV-visible absorption spectra were tested using a Shimadzu UV-2550 spectrophotometer.

1.4 Evaluation of Photocatalytic Activity

A 300 W xenon lamp was used as a simulated sunlight source, placed above the reaction solution. For the Cr(Ⅵ) reduction experiment, 16 mg of MIL-68(Fe)/g-C₃N₄ catalyst was mixed with 40 mL of a solution containing 15 mg/L Cr(Ⅵ) (potassium dichromate). For the tetracycline degradation experiment, 16 mg of MIL-68(Fe)/g-C₃N₄ catalyst was mixed with 40 mL of a solution containing 30 mg/L tetracycline. In the simultaneous removal experiment of Cr(Ⅵ) and tetracycline, 16 mg of the composite catalyst was mixed with 40 mL of a solution containing 30 mg/L tetracycline and 15 mg/L Cr(Ⅵ). Before starting the photocatalytic reaction, the solution was stirred in the dark for 40 minutes to reach adsorption equilibrium. During the entire reaction, 2 mL of the sample solution was taken every 20 minutes and filtered with a 0.22 µm filter head to remove adsorbed materials。Finally, the Cr(Ⅵ) content in the samples was determined by the diphenyl carbazide colorimetric method (Y. C. Zhang et al. 2011), and the absorbance at 356 nm was measured by a spectrophotometer to determine the tetracycline concentration in the samples. The reduction and degradation efficiency of Cr(Ⅵ) and tetracycline (mg/min) were calculated by Eq. (1):

$$\:R（\%）=\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\times\:100\%$$

where C₀ and C_t are the concentrations of the pollutant at 0 and t minutes of light exposure, respectively.

2.1 Characterizations

The SEM images of MIL-68(Fe), g-C₃N₄, and MIL-68(Fe)/g-C₃N₄ are shown in Fig. 2. The theoretical framework configuration of MIL-68(Fe) is a three-dimensional network, consisting of one-dimensional channels of triangular and hexagonal types along the c-axis. The SEM images reveal that the prepared MIL-68(Fe) is composed of large, coarse polyhedral prisms, consistent with previous reports(Liang et al. 2021). However, its average size reaches the micron scale (Fig. 2-1a), and such large dimensions could likely impact the catalytic performance of MIL-68(Fe).

In the SEM images of the MIL-68(Fe)/g-C₃N₄ composite, g-C₃N₄ and MIL-68(Fe) are observed to be closely integrated. Additionally, the size of MIL-68(Fe) in the MIL-68(Fe)/g-C₃N₄ composite is much smaller than that of the original MIL-68(Fe), reducing from the micron scale to the nanometer scale. This indicates that g-C₃N₄, as a precursor, plays a crucial role in regulating the formation of MIL-68(Fe) crystals during synthesis. Although the size of the composite material is reduced, all samples retain the three-dimensional network structure of MIL-68(Fe). The size reduction of MIL-68(Fe)/g-C₃N₄-3 is the most significant, showing a needle-like structure with a width of only a few tens of nanometers under the microscope. The reduction in size brought by the composite is beneficial for increasing the specific surface area of the material, thereby providing more exposed surfaces and active sites.

The elemental distribution on the surface of the materials was analyzed using EDS. Figure 3 shows the EDS elemental mapping of the representative composite material MIL-68(Fe)/g-C₃N₄-3. The elemental maps reveal the presence of four elements: C, N, O, and Fe. The brighter areas corresponding to Fe and O indicate higher concentrations, originating from MIL-68(Fe) on the surface of g-C₃N₄, while the weaker N signal comes from g-C₃N₄. These results demonstrate that MIL-68(Fe) is highly dispersed on the surface of the composite material, which is beneficial for exposing more active sites in the MIL-68(Fe)/g-C₃N₄-3 composite.

The XRD patterns of MIL-68(Fe), g-C₃N₄, and the prepared MIL-68(Fe)/g-C₃N₄ composites are shown in Fig. 4. The characteristic diffraction peaks of the MIL-68(Fe)/g-C₃N₄ composites align well with those reported for MIL-68(Fe)(Liang et al. 2017)and g-C₃N₄(H. Li et al. 2014), The intensity of the MIL-68(Fe) characteristic peaks in the composites is comparable to that of the original MIL-68(Fe), indicating that the crystal structure of MIL-68(Fe) is preserved in the composites. This is logical due to two reasons: (1) the percentage of g-C₃N₄ in the MIL-68(Fe)/g-C₃N₄ composites is relatively small, and (2) the in situ growth process ensures a uniform distribution of MIL-68(Fe) on g-C₃N₄, explaining the absence of g-C₃N₄ diffraction peaks in composites with low g-C₃N₄ content, such as MIL-68(Fe)/g-C₃N₄-3, MIL-68(Fe)/g-C₃N₄-4, and MIL-68(Fe)/g-C₃N₄-5. Additionally, a shift in the characteristic peaks of g-C₃N₄ in the composites suggests strong coupling interactions between g-C₃N₄ and MIL-68(Fe) during the formation of the heterojunction.

Figure 5 shows the FTIR spectra of MIL-68(Fe), g-C₃N₄, and MIL-68(Fe)/g-C₃N₄-3. The prepared MIL-68(Fe)/g-C₃N₄-3 exhibits similar characteristic peaks to MIL-68(Fe). The peak at 555 cm⁻¹ corresponds to the Fe-O stretching vibration, indicating the formation of metal-oxygen bonds between the carboxyl group of terephthalic acid and Fe(Ⅲ)(C.-C. Wang et al. 2016). The peak at 746 cm⁻¹ corresponds to the C-H bond vibration in the benzene ring. The absorption bands of carboxyl groups coordinated with the metal center (C = O, C-O) are located at 1545 cm⁻¹ and 1392 cm⁻¹(Giannakoudakis et al. 2017), respectively. Additionally, the characteristic peaks at 1236 cm⁻¹ and 1641 cm⁻¹ might overlap with some peaks of g-C₃N₄, causing a shift in the characteristic peaks of g-C₃N₄ in the composite. Therefore, the FTIR results further confirm the formation of strong interaction heterojunctions in the MIL-68(Fe)/g-C₃N₄-3 material.

Furthermore, the chemical composition of g-C₃N₄ nanosheets and original g-C₃N₄ was analyzed using XPS. According to the XPS spectra in Fig. 6, the MIL-101(Fe)/g-C₃N₄ heterostructure contains four elements: C, N, Fe, and O. The N 1s characteristic peaks in the range of 396–402 eV (Fig. 6b) further confirm the successful incorporation of g-C₃N₄ into the heterostructure. The two peaks at 400.8 and 398.6 eV in the N 1s spectrum correspond to sp² hybridized N atoms in C-N and C = N, respectively. In the C 1s spectrum (Fig. 6b), the peaks at 284.0 and 285.4 eV correspond to C-C and C-N groups in g-C₃N₄, and the peaks at 284.7 and 288.5 eV correspond to the benzoic acid and C = O bonds in H2BDC. In the O 1s spectrum (Fig. 6d), the peaks at 531.8 eV and 530.2 eV correspond to the oxygen components of terephthalic acid and the Fe-O bonds in MIL-68(Fe). The Fe 2p XPS spectrum (Fig. 6e) shows two peaks at 711.9 and 725.6 eV, corresponding to Fe 2p₃/₂ and Fe 2p₁/₂, respectively. The BE difference of 13.7 eV between these peaks indicates that Fe is in the + 3 oxidation state, which is further supported by the peak at 716.4 eV.

The thermal stability of MIL-68(Fe)/g-C₃N₄-3, MIL-68(Fe), and g-C₃N₄ in a nitrogen atmosphere was investigated using thermogravimetric analysis (TGA). As shown in the TGA curves in Fig. 7b, the original MIL-68(Fe) exhibited only minimal weight loss in the temperature range of 25–300°C, but significant weight loss occurred at temperatures above 300°C, indicating that MIL-68(Fe) does not possess particularly good thermal stability. Pure g-C₃N₄ showed very slight weight loss throughout the entire temperature range, with noticeable weight loss starting above 600°C, demonstrating good thermal stability. Given the proportion of g-C₃N₄ added in the preparation method, g-C₃N₄ constitutes only about 1% of MIL-68(Fe)/g-C₃N₄-3, so the weight retention of pure g-C₃N₄ has a negligible impact on the weight change of MIL-68(Fe)/g-C₃N₄-3 upon heating. MIL-68(Fe)/g-C₃N₄-3 experienced minimal weight loss in the temperature range of 25–420°C, with significant weight loss starting above 420°C. After the composite material was formed, MIL-68(Fe)/g-C₃N₄-3 exhibited better thermal stability than the original MIL-68(Fe).

The UV-vis characterization of MIL-68(Fe)/g-C₃N₄-3 and MIL-68(Fe) (Fig. 7a) shows that MIL-68(Fe)/g-C₃N₄-3 has a UV-vis spectrum that highly overlaps with that of MIL-68(Fe), indicating that the optical properties of g-C₃N₄ nanosheets did not change. This preservation of the optical properties is beneficial for maintaining the photocatalytic performance of MIL-68(Fe)/g-C₃N₄-3.

2.2 Photocatalytic performance

The prepared MIL-68(Fe)/g-C₃N₄ material was used as a photocatalyst for pollutant removal, and its photocatalytic performance was investigated. First, the photocatalytic reduction of Cr(Ⅵ) by MIL-68(Fe)/g-C₃N₄ was studied. To eliminate potential effects from hydrolysis or photolysis, control experiments were conducted. As shown in Fig. 8, the concentration of Cr(Ⅵ) did not decrease in the absence of a catalyst, indicating that Cr(Ⅵ) alone does not react under sunlight irradiation without a photocatalyst.

Before conducting the photocatalytic reaction with the catalyst, the adsorption of Cr(Ⅵ) by the photocatalyst in the dark was explored. Under dark conditions, g-C₃N₄, MIL-68(Fe), and MIL-68(Fe)/g-C₃N₄ could all adsorb small amounts of Cr(Ⅵ) and reached equilibrium within 40 minutes. According to previous research, MIL-68 achieved a Cr(Ⅵ) removal rate of 76.0% when ammonium oxalate was used as a scavenger, but only an 8% reduction rate for Cr(Ⅵ) without the scavenger(Jing et al. 2017). In our experiments, which did not use a scavenger, MIL-68(Fe) achieved a similarly low Cr(Ⅵ) reduction rate of 8%. g-C₃N₄ also exhibited a low Cr(Ⅵ) reduction rate, consistent with previous reports.

In contrast, the MIL-68(Fe)/g-C₃N₄ heterojunction photocatalyst demonstrated higher photocatalytic activity for Cr(Ⅵ) reduction compared to the individual materials. Among all the photocatalysts, MIL-68(Fe)/g-C₃N₄-3 showed the highest Cr(Ⅵ) removal efficiency, reducing the Cr(Ⅵ) concentration by 99.9% after 80 minutes of reaction. This high efficiency is attributed to the small needle-like morphology and large specific surface area of MIL-68(Fe)/g-C₃N₄-3.

The photocatalytic reduction kinetics of Cr(Ⅵ) are shown in Fig. 9. and the samples followed a first-order kinetic model. The study of photocatalytic Cr(Ⅵ) reduction kinetics revealed that the reduction rate of Cr(Ⅵ) by MIL-68(Fe)/g-C₃N₄-3 was 0.04519 min⁻¹, which is approximately 376 times higher than that of MIL-68(Fe) alone (0.00012 min⁻¹) and 594 times higher than that of g-C₃N₄ alone (0.000076 min⁻¹). Additionally, the physical mixture of MIL-68(Fe) and g-C₃N₄ exhibited poor Cr(Ⅵ) reduction efficiency, significantly lower than that of MIL-68(Fe)/g-C₃N₄-3. These results indicate that the introduction of g-C₃N₄ into MIL-68(Fe) significantly enhances the photocatalytic activity of MIL-68(Fe)/g-C₃N₄-3.

The photocatalytic degradation of tetracycline by MIL-68(Fe)/g-C₃N₄ was also tested. To eliminate the possibility of natural photolysis, control experiments were conducted. As shown in Fig. 10, the concentration of tetracycline did not decrease under light irradiation without a photocatalyst, indicating that light alone does not degrade tetracycline. Before starting the photocatalytic degradation reaction, the adsorption of tetracycline by the photocatalysts in the dark was investigated. Under dark conditions, all the catalysts adsorbed tetracycline within 40 minutes, with MIL-68(Fe)/g-C₃N₄-3 showing the highest adsorption amount due to its small needle-like morphology and large specific surface area.

When using the original MIL-68(Fe) as a photocatalyst, the degradation rate of tetracycline was only 29.6%, indicating weak photocatalytic degradation ability. The g-C₃N₄ photocatalyst exhibited a high photocatalytic activity for tetracycline degradation, achieving a degradation efficiency of 92% after 40 minutes of light irradiation. In comparison, the degradation rate of tetracycline by MIL-68(Fe)/g-C₃N₄ was significantly higher than that of MIL-68(Fe) alone but did not exceed that of g-C₃N₄ alone.

According to the synthesis method of the composite materials, the amount of g-C₃N₄ added was very small, approximately 1% in MIL-68(Fe)/g-C₃N₄-3. Therefore, the catalytic ability of g-C₃N₄ alone has a negligible impact on the catalytic performance of MIL-68(Fe)/g-C₃N₄-3. The enhanced performance of MIL-68(Fe)/g-C₃N₄ in photocatalytic degradation of tetracycline is not due to the excellent degradation ability of g-C₃N₄ itself but rather because the incorporation of g-C₃N₄ into MIL-68(Fe) forms a scaffold structure that improves the morphology and specific surface area, thereby accelerating the photocatalytic degradation of tetracycline.

Among the MIL-68(Fe)/g-C₃N₄ composites, MIL-68(Fe)/g-C₃N₄-3 showed the highest removal efficiency for tetracycline, achieving a degradation rate of 90% after 80 minutes of light irradiation. The optimal photocatalytic degradation result of tetracycline by MIL-68(Fe)/g-C₃N₄-3 is consistent with the results of photocatalytic reduction of Cr(Ⅵ). MIL-68(Fe)/g-C₃N₄-3 can be considered the optimal composite material for photocatalytic removal of pollutants.

In actual industrial environments, wastewater often contains heavy metals along with other harmful organic pollutants. The simultaneous oxidation and reduction reactions of photocatalysts to remove pollutants are of significant importance in industrial applications. As shown in Fig. 12a, in the absence of a photocatalyst, the concentrations of tetracycline and Cr(Ⅵ) remained essentially unchanged after 160 minutes of light irradiation when both were present. However, when MIL-68(Fe)/g-C₃N₄-3 was used as a photocatalyst, the reduction efficiency of Cr(Ⅵ) greatly increased in the presence of tetracycline, with Cr(Ⅵ) almost completely reduced after 40 minutes of light irradiation. In the absence of tetracycline, the reduction rate of Cr(Ⅵ) was only 48%

As shown in Fig. 12b, the presence of Cr(Ⅵ) also significantly improved the degradation efficiency of tetracycline by MIL-68(Fe)/g-C₃N₄-3. After 40 minutes of reaction, the removal rate of tetracycline reached 70%, compared to only 53% in the absence of Cr(Ⅵ) under the same reaction time. However, as the reaction proceeded and Cr(Ⅵ) was nearly completely reduced, the reaction rate in the group with added Cr(Ⅵ) slowed down, and eventually, both groups achieved approximately the same removal rate of tetracycline.

These results highlight the enhanced photocatalytic performance of MIL-68(Fe)/g-C₃N₄-3 in the simultaneous removal of both Cr(Ⅵ) and tetracycline, demonstrating its potential effectiveness in treating industrial wastewater containing multiple types of contaminants.

The stability of photocatalysts is crucial for practical applications. Therefore, MIL-68(Fe)/g-C₃N₄-3 was subjected to five consecutive cycles of photocatalytic removal of Cr(VI) in a coexistence environment to assess its stability. As shown in Fig. 13, MIL-68(Fe)/g-C₃N₄-3 maintained good photocatalytic activity after five cycles. Interestingly, the reduction efficiency of Cr(VI) by MIL-68(Fe)/g-C₃N₄-3 did not change significantly during the first three cycles but slightly decreased in the last two cycles. This could be due to Cr(III) adsorbing onto the surface of MIL-68(Fe)/g-C₃N₄-3 during the previous reaction, where the disproportionation reaction between adsorbed Cr(III) and Cr(VI) accelerated the reduction of Cr(VI). However, as the number of cycles increased, the excessive adsorption of Cr(III) on the surface of MIL-68(Fe)/g-C₃N₄-3 covered the active sites.

The chemical stability of the recovered MIL-68(Fe)/g-C₃N₄-3 was also tested using XRD. In Fig. 14a. the XRD patterns of MIL-68(Fe)/g-C₃N₄-3 showed little change after five cycles, indicating that MIL-68(Fe)/g-C₃N₄-3 has good chemical stability. In Fig. 14b SEM images further confirmed that MIL-68(Fe)/g-C₃N₄-3 maintained its structure well with no significant changes after five cycles.

2.3 Exploration of the possible Mechanisms for Enhanced Photocatalytic Performance

As shown in Fig. 16, the N2 adsorption/desorption isotherms and pore size distribution of the samples were analyzed. According to previous studies (Tan et al. 2017), pure g-C₃N₄, being a non-porous material, exhibits relatively low specific surface area and porosity. The original MIL-68(Fe) also showed a relatively low specific surface area and pore size (SBET 186.9 m²/g). Compared to other Fe-MOFs, MIL-68(Fe) has a lower specific surface area due to its bulky structure, limiting its catalytic applications.

After forming the composite, MIL-68(Fe)/g-C₃N₄-3 exhibited a nearly five-fold increase in specific surface area compared to MIL-68(Fe), consistent with the reduced size of the composite observed in SEM characterization. g-C₃N₄ provides a template for the growth of MIL-68(Fe), which cuts the bulky structure of MIL-68(Fe), enhancing the exposure of the porous layers within the crystal. Some Fe-MOF/g-C₃N₄ composites have shown decreased specific surface areas after formation, such as MIL-100(Fe)/g-C₃N₄ (from 198.7 m²/g to 131.6 m²/g) and MIL-53(Fe)/g-C₃N₄ (from 20.6 m²/g to 18.5 m²/g). However, the synthesized MIL-68(Fe)/g-C₃N₄-3 displayed an impressive increase in specific surface area (from 186.9 m²/g to 1046.8 m²/g). This significant increase in specific surface area is attributed to the g-C₃N₄'s cutting effect, which prevents the bulk accumulation of MIL-68(Fe) and promotes the growth of smaller crystals without destroying the basic structure of MIL-68(Fe).

The light absorption properties of semiconductors determine their photocatalytic activity. Therefore, UV-DRS analysis of MIL-68(Fe) and MIL-68(Fe)/g-C₃N₄-3 was conducted, as shown in Fig. 15. The original MIL-68(Fe) can absorb visible light, appearing orange. The calculated band gap of MIL-68(Fe) from the Kubelka-Munk plot is 2.18 eV. After doping with g-C₃N₄, the UV absorption of MIL-68(Fe)/g-C₃N₄-3 decreased. The absorption edge of the composite showed a red shift, indicating that the band gap of the MIL-68(Fe)/g-C₃N₄ heterostructure narrowed. The band gap of MIL-68(Fe)/g-C₃N₄-3, calculated from the Kubelka-Munk plot (Fig. 15b), is 1.98 eV, suggesting that MIL-68(Fe)/g-C₃N₄-3 may have better visible light response. Similar band gap reductions have been observed in other MOFs heterojunctions with rGO(Yang et al. 2017)和g-C₃N₄(Huang et al. 2017), indicating that narrower band gaps facilitate improved photocatalytic efficiency.

In this study, we designed a method for the in-situ growth synthesis of MIL-68(Fe)/g-C₃N₄ composites, which exhibit excellent heterostructures. Moreover, the template framework of g-C₃N₄ plays a positive role in reducing the bulky volume of MIL-68(Fe). SEM images show a significant reduction in the micro-morphological volume of MIL-68(Fe)/g-C₃N₄ composites, facilitating the exposure of active sites within the crystal. Results from XRD, FTIR, and XPS indicate that the crystal structure, groups, and chemical composition of MIL-68(Fe) in the composites remain unchanged, which is crucial for maintaining the inherent properties of MIL-68(Fe). Compared to individual MIL-68(Fe) and g-C₃N₄, the MIL-68(Fe)/g-C₃N₄ composites demonstrate superior photocatalytic activity. The optimal MIL-68(Fe)/g-C₃N₄-3 shows a Cr(VI) reduction rate up to 376 times that of MIL-68(Fe) and a tetracycline degradation rate up to 3 times that of MIL-68(Fe). Additionally, in the presence of both tetracycline and Cr(VI), the removal efficiency of MIL-68(Fe)/g-C₃N₄-3 for these pollutants further improves. BET results reveal that the specific surface area of MIL-68(Fe)/g-C₃N₄ composites significantly increases from 186.9 m²/g to 1046.8 m²/g, a more than fivefold enhancement. UV-DRS analysis shows that MIL-68(Fe)/g-C₃N₄ has a narrower bandgap compared to MIL-68(Fe), further elucidating the superior photocatalytic performance of MIL-68(Fe)/g-C₃N₄. The reduction in size of MIL-68(Fe)/g-C₃N₄ composites, resulting in increased specific surface area, allows for more active site exposure, promoting photocatalytic reactions on the catalyst surface.

Acknowledgments

This work was supported by the Guangzhou Key R&D Program Project (2021A05202) and Ministry of Science and Technology of China for State Key Research and Development Project (2023S017082).

Funding

This work was supported by the Guangzhou Key R&D Program Project (2024B03J1312), and the Maoming Science and Technology Plan Project (2023S017082).

Author information

School of Food Science & Engineering, South China University of Technology, Guangzhou, 510640, Guangdong, China

Fenglei Liu, Kai Chen, Liang Zhu, Kaijun Xiao

School of Environment and Energy, South China University of Technology, Guangzhou 510006, China

Zhaocai Shi

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Fenglei Liu contributed to conceptualization, methodology, and software. Kai Chen contributed to writing which included review and editing. Kaijun Xiao and Zhaocai Shi contributed to supervision, project administration, and funding acquisition. All authors contributed to the study conception and design. All authors read and approved the final manuscript.

Ethics approval

This article does not contain any study with human and animals performed by any of the author. Consent to participate is not applicable.

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Written informed consent was obtained from the parents.

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The participant has consented to the submission of the study to the journal.

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Research on the Preparation of MIL-68(Fe)/g-C₃N₄ Materials for the Simultaneous Photocatalytic Removal of Cr(Ⅵ) and Tetracycline

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Abstract

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Introduction

1. Experimental

1.1 Materials

1.2 Synthesis of Photocatalytic Samples

1.3 Characterization

1.4 Evaluation of Photocatalytic Activity

2. Results and discussion

2.1 Characterizations

2.2 Photocatalytic performance

2.3 Exploration of the possible Mechanisms for Enhanced Photocatalytic Performance

3. Conclusions

Declarations

References

Additional Declarations

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